The present invention relates to an air-fuel ratio control apparatus for internal combustion engines and more particularly to such air-fuel ratio control apparatus including means for calibrating the output characteristic of an air-fuel ratio sensor against its change with time due to any dirt or pollution of the air-fuel ratio sensor.
In a known apparatus of this type such as disclosed in JP-A-58-57050, there is provided a discriminating unit which utilizes the atmosphere as a known air-fuel ratio for the calibration of a variation with time of the output characteristic of an air-fuel ratio so as to determine whether the surrounding of the sensor is filled with the air whereby in response to the discriminating unit, a correction factor for the air-fuel ratio sensor output characteristic is calculated and the desired calibration is effected when the surrounding of the air-fuel ratio sensor is filled with the air.
In the above-mentioned conventional technique, since there is practically no chance of the sensor surrounding being filled with the air, a situation is caused where the output characteristic of the sensor is not calibrated in a long period of time. There is a problem especially in the case of an automobile equipped with an automatic transmission or in the case of an automobile driven by the driver without using engine braking.